Jet outflow and open field line measurements on the C-2W advanced beam-driven field-reversed configuration plasma experiment

REVIEW OF SCIENTIFIC INSTRUMENTS 89, 10D120 (2018)
Jet outflow and open field line measurements on the C-2W advanced
beam-driven field-reversed configuration plasma experiment
D. Sheftman,1,a) L. Schmitz,1,2 D. Gupta,1 M. C. Thompson,1 and TAE Team1,b) 1TAE Technologies, Inc., P.O. Box 7010, Rancho Santa Margarita, California 92688-7010, USA 2Department of Physics and Astronomy, University of California Los Angeles, Los Angeles, California 90095, USA
(Presented 17 April 2018; received 20 April 2018; accepted 22 June 2018; published online 3 October 2018)
Accurate operation and high performance of the open field line plasma surrounding the field reversed configuration is crucial to achieving the goals of successful temperature ramp up and confinement improvement on C-2W. Knowledge and control of the open field line plasma requires extensive diagnostic efforts. A suite of diagnostics, which consists of microwave interferometry, dispersive spectroscopy, and spatial heterodyne spectroscopy, is being developed to measure electron den- sity, ion temperature, and particle outflow velocity at various locations along the open magnetic field lines. A detailed overview of these diagnostics is presented. Published by AIP Publishing. https://doi.org/10.1063/1.5037114
I. INTRODUCTION
Experiments are being carried out on C-2W, an axis- symmetric Field Reversed Configuration (FRC) plasma appa-
1
ogy of the C-2U apparatus,2 designed to sustain a plasmoid of
3.2 m length, 0.4 m radius, 5 × 1013 cm−3 density, and 3 keV
total temperature, for a duration of 30 ms. The goal of the
experiment is to successfully ramp up electron temperature and
demonstrate the scalability of the plasma temperature in the
path toward fusion grade FRC devices. It has been shown that
control of the end loss plasma (plasma lost to the divertor elec-
trodes) is crucial for the increase of particle and energy decay
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importance to the successful operation of the C-2W experi- ment. A diverse suite of diagnostics was developed and will be installed on C-2W to measure the properties of the jet plasma. These diagnostic methods will provide measurements of elec- tron density, electron temperature, and ion temperature and outflow velocity at various locations along the open magnetic field line region. Three of these diagnostics are presented in this paper.
II. DIAGNOSTIC METHODS A. Overview
The open field line region on C-2W consists of the plasma outside the closed magnetic field lines of the FRC core. This includes the outer divertor, formation section, inner divertor, mirror, and outer parts of the confinement vessel. The flow of plasma from the FRC core until termination is as follows: Plasma diffuses radially outside the separatrix of the FRC into the jet. The plasma then flows axially toward the mirror sec- tions at the edge of the confinement vessel. Particles with velocities within the loss cone are able to pass through the magnetic field gradient at the mirror and later accelerate in the expansion field of the inner divertor. The magnetic field ratio between the mirror and confinement sections is <3, whereas the ratio between mirror section and inner divertor at the elec- trode plates can reach up to 30. Thus, particles may accelerate in the inner divertor beyond the ion sound velocity. Many par- ticles from the FRC terminate at the inner divertor electrode plates, while the rest of the particles exit the inner divertor, flow through the formation section, and terminate at the outer divertor electrode plates. Several diagnostic instruments mea- sure the plasma flowing from confinement to termination at the inner divertor (see Fig. 1), while the behavior of the plasma terminating at the outer divertor is not discussed here and is beyond the scope of this paper.
ratus. C-2Wisanadvancedexperimentbasedonthetechnol-
times.
ried out on C-2U,4 showing a stable, low velocity jet outflow. The low velocity at Mach number M < 0.1 can be explained by the high amplitude of the magnetic field of the mirror plugs, which were located toward the end loss electrodes in the expansion divertors. These mirror plugs were designed specifically to limit the end loss of particles from the FRC plasma. One difference between C-2W and the predecessor, C-2U, is the introduction of inner divertors; see Fig. 1. The inner divertors are a pair of large plasma expansion vessels containing the end loss electrodes, while the outer divertors are similar expansion vessels located at the machine edges and serve to terminate the exhaust from the formation process and leftover jet outflow from the FRC core. This scenario intro- duces new physics to the open field line plasma (henceforth referred to as the “jet”) region, not encountered in previous FRC or open field mirror experiments. Therefore, knowledge and control of the behavior of the jet plasma is of crucial
Measurements of the end loss plasma have been car-
Note: Paper published as part of the Proceedings of the 22nd Topical Conference on High-Temperature Plasma Diagnostics, San Diego, California, April 2018.
a) dsheftman@tae.com
b)TAE Team members are listed in Nucl. Fusion 57, 116021 (2017). 0034-6748/2018/89(10)/10D120/5/$30.00
89, 10D120-1 Published by AIP Publishing.